wow this is honestly amazing the only things that I saw really is the shirt. on the neck lining of the shirt you should try and add the slightest of curve to it so it looks like the shirt is more 3dimesional (if that makes any sense)
the other thing I saw is you need to add where the light hits on the hair or more accurately the highlighted parts on his hair. those are really the only things I could see that you can on a bit, but can I just it is amazing. honestly I think you shaded everything really amazing and even caught his facial expression amazingly and don't even get me started on how great those hands are. you are a great artist and if you draw anything else you see, and now I'm rambling so I should probably go now keep up the good work